Each pixel is assigned a numerical value (CT number), that is the average of all the attenuation values pertained within the corresponding voxel. This number is compared to the attenuation value of water and displayed on a scale of arbitrary units named Hounsfield units (HU) after Sir Godfrey Hounsfield.

This scale assigns water as an attenuation value (HU) of zero. The range of CT numbers is 2000 HU wide although modern scanners have a greater range of HU up to 4000. Each numeral value represents a shade of grey with +1000 (white) and –1000 (black) at either end of the spectrum.